The present invention is directed, in general, to liquid crystal displays and, more specifically, to reflective, transmissive and transflective black-and-white and color cholesteric liquid crystal displays.
The development of improved low-power-consumption flat-panel liquid crystal displays (LCDs) is an area of very active research, driven in large part by the proliferation of and demand for portable electronic appliances, including computers and wireless telecommunications devices. Moreover, as the quality of LCDs improve, and the cost of manufacturing declines, it is projected that LCDs may eventually displace conventional display technologies, such as cathode-ray-tubes.
Cholesteric liquid crystal (xe2x80x9cCLCxe2x80x9d) technology is a particularly attractive candidate for many display applications. CLC displays can be used to provide bi-stable and multi-stable displays that, due to their non-volatile xe2x80x9cmemoryxe2x80x9d characteristic, do not require a continuous driving circuit to maintain a display image, thereby significantly reducing power consumption. Moreover, some CLC displays can be easily viewed in ambient light without the need for back-lighting; such displays are referred to as xe2x80x9creflectivexe2x80x9d mode displays, while those requiring a back-light are referred to as xe2x80x9ctransmissivexe2x80x9d mode displays. The elimination of the need for back-lighting is particularly significant in that lighting requirements typically represent approximately 90% of the total power consumption of conventional LC displays. While a reflective mode display is suitable for some applications, and a transmissive mode display is suitable for others, there are certain applications in which it can also be desirable to have a display operable in both reflective and transmissive modes. Therefore, to meet the growing demand for LCDs, there is a need in the art for LCDs operable in reflective, transmissive, and selectable transmissive/reflective modes.
To address the above-described deficiencies of the prior art, the present invention provides various reverse-mode direct-view liquid crystal displays employing a liquid crystal having a characteristic wavelength to reflect light in the non-visible spectrum, including reflective, transmissive and reflective/transmissive mode displays. In accordance with the principles disclosed, the basic structure of a direct-view liquid crystal display (LCD) includes a front substantially transparent substrate, a rear substrate, and a controllable cholesteric liquid crystal (CLC) disposed between the front and rear substrates and having a characteristic wavelength to reflect light in the non-visible spectrum. Portions of the controllable CLC can selectively exhibit a planar state or a focal-conic state; the portions of the CLC in the planar state appear dark (e.g., black), and the portions of the CLC in the focal-conic state appear bright (e.g., white), to an observer of the LCD. The various reflective, transmissive and reflective-transmissive mode displays are derived by further combinations with linear, circular or near circular polarizers, reflective mirrors and/or transmissive/reflective (i.e., xe2x80x9ctransflectivexe2x80x9d) mirrors, color matrix reflectors or filters and a source of visible light.
In accordance with the principles of the invention, a direct-view LCD operative in a transmissive mode is constructed by combining with the basic structure a first linear polarizer disposed proximate the front of the LCD; a second linear polarizer disposed proximate the rear of the LCD, the polarity of the second linear polarizer being perpendicular, or nearly perpendicular, to the polarity of the first linear polarizer; and a source of visible light disposed proximate the rear of the LCD, the second linear polarizer being disposed intermediate to the CLC and the source of visible light and operative to linearly polarize the visible light prior to passage of the visible light through the CLC. In operation, i) the portions of the CLC in the planar state allow the transmission of substantially all of the linearly-polarized visible light, which is substantially blocked from view to an observer by the first linear polarizer, whereby the portions of the CLC in the planar state appear dark to an observer of the LCD; and ii) the portions of the CLC in the focal-conic state at least partially alter the polarization of the visible light, causing a portion of the visible light to have a linear polarization parallel to the polarity of, and transmittable through, the first linear polarizer, whereby the portions of the CLC in the focal-conic state appear bright to an observer of the LCD.
In accordance with the principles of the invention, a direct-view LCD operative in a reflective mode is constructed by combining with the basic structure a mirror disposed proximate the rear of the LCD, and a circular, or substantially circular, polarizer disposed proximate the front of the LCD, the circular polarizer operative to at least substantially circularly polarize visible light incident on the front of the LCD. In operation, i) the portions of the CLC in the planar state allow the transmission of substantially all of the circularly-polarized visible light, which is substantially reflected from the mirror and back through the CLC, the reflected visible light having a handedness opposite to that of, and thereby blocked by, the circular polarizer, whereby the portions of the CLC in the planar state appear dark to an observer of the LCD; and ii) the portions of the CLC in the focal-conic state at least partially alter the polarization of the circularly-polarized visible light prior and subsequent to being reflected by the mirror, causing a portion of the visible light to have a circular polarization of the same handedness as, and thus transmittable through, the circular polarizer, whereby the portions of the CLC in the focal-conic state appear bright to an observer of the LCD.
In accordance with the principles of the invention, a direct-view LCD operative in a reflective or transmissive mode is constructed by combining with the basic structure a first circular, or substantially circular, polarizer disposed proximate the front of the CLC, the first circular polarizer having a first handedness; a transmissive/reflective (or xe2x80x9ctransflectivexe2x80x9d) mirror disposed proximate the rear of the CLC, the transmissive/reflective mirror operative to reflect visible light incident from the front of the LCD and to transmit visible light incident from the rear of the LCD; a second circular, or substantially circular, polarizer disposed proximate the rear of the transflective mirror, the second circular polarizer having a second handedness opposite to the first handedness of the first circular polarizer; and a source of visible light disposed proximate the rear of the second circular polarizer.
In the transmissive mode of operation, the source of visible light is energized, and the second circular polarizer substantially circularly polarizes the visible light. The portions of the CLC in the planar state allow the transmission of substantially all of the circularly-polarized visible light, the circularly -polarized visible light having a handedness opposite to that of, and thereby blocked by, the first circular polarizer, whereby the portions of the CLC in the planar state appear dark to an observer of the LCD. The portions of the CLC in the focal-conic state at least partially alter the polarization of the circularly-polarized visible light, causing a portion of the visible light to have a circular polarization of the same handedness as, and thus transmittable through, the first circular polarizer, whereby the portions of the CLC in the focal-conic state appear bright to an observer of the LCD.
In the reflective mode of operation, the source of visible light is de-energized. The first circular, or substantially circular, polarizer is operative to substantially circularly polarize visible light incident on the front of the LCD. The portions of the CLC in the planar state allow the transmission of substantially all of the circularly-polarized visible light, which is substantially reflected from the transmissive/reflective mirror and back through the CLC, the reflected visible light having a handedness opposite to that of, and thereby blocked by, the first circular polarizer, whereby the portions of the CLC in the planar state appear dark to an observer of the LCD. The portions of the CLC in the focal-conic state at least partially alter the polarization of the circularly-polarized visible light prior and subsequent to being reflected by the transmissive/reflective mirror, causing a portion of the visible light to have a circular polarization of the same handedness as, and thus transmittable through, the first circular polarizer, whereby the portions of the CLC in the focal-conic state appear substantially bright to an observer of the LCD.
The use of a CLC having a characteristic wavelength to reflect non-visible spectrum can provide several advantages over conventional direct-view displays that utilize a CLC having a characteristic wavelength to reflect visible spectrum. For example, a smaller cell gap can be utilized, which reduces the driving voltage necessary to untwist the LC helical structure. The voltage necessary to untwist a CLC helical structure is, in part, a function of the pitch length of the CLC domains; the longer the pitch length, the lower the required driving voltage. Whereas a CLC having a characteristic wavelength to reflect non-visible infra-red spectrum has a longer pitch length that a CLC having a characteristic wavelength to reflect visible spectrum, a lower driving voltage is required. Furthermore, the use of a CLC having a characteristic wavelength to reflect non-visible spectrum yields a display that doesn""t depend on multiple LC planar layers for reflection. The bright state of the display depends only on the retarding and depolarizing effect of the CLC in the focal-conic state, which can be achieved with a smaller cell gap (potentially as small as 1 micron) than conventional displays using a CLC having a characteristic wavelength to reflect visible spectrum, which depend on multiple LC planar layers for reflection to yield a bright state.
The foregoing has outlined rather broadly the features and technical advantages of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features and advantages of the invention will be described hereinafter that form the subject matter of the claims recited hereinafter. Those skilled in the art should appreciate that they may readily use the conception and the specific embodiments disclosed as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form, except as limited by the claims recited hereinafter.